Starchy foods more filling than fiber, lab tests suggest

Moving away from grass-heavy ancestral diets may not be the reason for obesity epidemic

GRASSY DIET Gelada baboons have a grass-based diet like that eaten by some human ancestors. But tests of gut microbes from the baboons and from humans suggest that a fiber-filled diet produces fewer appetite-curbing hormones than a potato-based one.

Potato starch may be more filling than fiber, a showdown in an artificial colon indicates. The finding may challenge the idea that the worldwide obesity epidemic is a side effect of eating starchy foods rather than the high-fiber plants human ancestors evolved with.

The results are “interesting, but not ready for direct application to people,” says David Ludwig, a pediatric endocrinologist at Boston Children’s Hospital. The artificial colons are missing many pieces — such as a brain, immune system and endocrine system — that work together to control eating habits and body weight, he says. For instance, the test tube system used in the study didn’t factor in the hormone insulin, which Ludwig and others think is a major player in regulating body weight.

Ancestral diets were much higher in fiber, especially indigestible plants fibers, than most modern human diets are. Human stomachs and small intestines can’t break down lignin and cellulose in plant fiber; instead the microbes that live in people’s colons ferment the fiber to make short chain fatty acids and other energy-rich chemicals that the human body can absorb. These short chain fatty acids trigger production of appetite-suppressing hormones.

Because starchy plants like potatoes don’t contain lignin and are easier to digest, scientists have assumed that gut microbes don’t produce as many short chain fatty acids from these foods. Without these chemicals and the appetite-curbing hormones they stimulate, people would probably eat more of the calorie-laden food and gain weight, scientists reasoned.

Microbiologist Glenn Gibson of the University of Reading in England and dietician Gary Frost of Imperial College London and their colleagues tested the theory by feeding gut microbes either potato starch or fiber from grasses called fescue (Festuca rubra) to see which diet would lead to more appetite-suppressing chemicals.

The researchers collected gut microbes from three human vegetarians and from three gelada baboons. Gelada baboons are the only primates that have a grass-based diet like that eaten by many human ancestors, Frost says.

Researchers in Gibson’s lab put microbes from each person or baboon into three flasks that mimicked different conditions in the colon. One flask contained potato starch that had been predigested as it would be by the stomach and small intestines. Another flask held predigested grass leaves. And a control flask mimicked a diet low in indigestible starch or fiber, giving the bacteria little to ferment.

Both the human and gelada gut microbes produced more short chain fatty acids, such as acetate, lactate and butyrate, from potato starch than from the high-fiber diet, the researchers report May 20 in mBio. The fatty acids from the potato diet also stimulated colon cells grown in lab dishes to release more of an appetite-suppressing hormone called peptide YY than the grass diet did.

It was surprising to find that a high-fiber diet didn’t result in more appetite-curbing hormones, Gibson says. The study tested only the effect of carbohydrates, but the researchers have other data suggesting that protein content may be more important for controlling appetite, he says.

Although the study highlights the effect diet can have on the gut microbiome, the collection of bacteria and other microbes that live in the intestines, it doesn’t directly contradict years of studies in living people that suggest high-fiber diets are beneficial to health and weight control, Ludwig says.